This project is designed to continue the study of the similarities and differences in the interaction of estrogens and antiestrogens with uterine tissue, and in particular, to distinguish the subcellular site or sites where estrogens and antiestrogens exhibit clearly differing actions. This subcellular site(s) may then be the point at which antiestrogens inhibit any further estrogenic response. In order to determine why the initial interaction of estrogens versus antiestrogens with the cytoplasmic estrogen receptor and the subsequent formation of the nuclear estrogen receptor results in differential nuclear binding, the interaction of these receptor complexes to the acceptor-effector site(s) on chromatin will be investigated. This will be done by studying the receptor complex binding to isolated and purified chromatin subfractions. Such studies will require the synthesis of radiolabeled high affinity antiestrogens. Structural differences between the estrogen and antiestrogen receptor complexes (cytoplasmic or nuclear) will be studied primarily by gel filtration and ion exchange chromatography. In addition to using 3H-labeled estrogens or antiestrogens, the receptor itself will be labeled with 65Zn. Consideration will be given to the nature of the in vivo salt-extractable versus salt-resistant nuclear binding of antiestrogen and estrogens and the relationship of such binding to the "fallout" and recycling of the nuclear estrogen receptor.